UMR 7204 Centre des sciences de la conservation
Actuarial senescence increases the risk of extinction of mammal populations
Author(s): Chantepie, S, Téplitsky, C, Sarrazin, F, Robert, A
Despite recent general acknowledgement that senescence has significant negative demographic manifestations in natural environments across a wide range of animal species, we still do not know if it can significantly impact the dynamics and viability of wild populations. Focusing on actuarial senescence (i.e., the decline of survival probabilities at old ages), we use specific demographic information to project the dynamics and extinction risk of wild populations of 58 species of mammals in the presence or absence of senescence. Our projections reveal major negative effects of ageing on population dynamics and viability, with a 27% decrease of the time to extinction on average and a potential deterioration of the species projected conservation status in 10% of the cases. The magnitude of the senescence cost is relatively homogenous among mammal orders at the exception of primates, which are disproportionally affected, due to their slow pace of life. Senescence is associated with particularly strong increases of the extinction risk in species with high annual survival probabilities of young adults and long intervals between litters, independently of their place in the phylogeny, indicating that the pace of life history can be used to forecast the detrimental effects of ageing on the viability of species.
Department of Integrative Biology
Demographic heterogeneity, selection, and population response
Author(s): Fox, GA, Kendall, BE
Demographic heterogeneity (unmodeled variation in traits underlying vital rates) has attracted much attention in recent years. Empirical studies show substantial heterogeneity in many populations. Theory shows that it can have strong impact on demographic variance (and by inference, extinction risk); some kinds of demographic heterogeneity can also have large effects on the mean population growth rate.
Our studies of heterogeneity point to a strong connection with natural selection, and provide new insights on how changes in phenotype distributions can occur in structured populations. The key is an understanding of the within-population phenotypic correlation structure. Correlations can occur within individuals over time, between individuals at a given time, and between individuals at different times.
In matrix models allowing persistent heterogeneity, survival heterogeneity increases the mean population growth rate, and the population becomes dominated by "good survivors" even if the parent-offspring correlation is zero or negative. By contrast, the effect of fertility heterogeneity depends on the sign of the parent-offspring correlation. This is because information transmission across time depends (for reproduction) entirely on the parent-offspring correlation, but for survival information is also transmitted by changes in the population's phenotype structure.
With overlapping generations, then, selection on survival and on reproduction can cause different responses. The demographic response (change in stable phenotype distribution) occurs because of the non-genetic correlation structure of the population. For survival, this response occurs regardless of the parent-offspring correlation, but for reproduction, the response depends strongly on the parent-offspring correlation. This suggests that selection on survival may often lead to larger responses and faster changes than is the case for selection on reproduction.
Department of Biology
Estimating fluctuating selection in age-structured populations
Author(s): Kvalnes, T, Engen, S, Sæther, B, Jensen, H
In age-structured populations, viability and fecundity selection of varying strength may occur in different age classes. On the basis of an original idea by Fisher of weighting individuals by their reproductive value, we show that the combined effect of selection on traits at different ages acts through the individual reproductive value defined as the stochastic contribution of an individual to the total reproductive value of the population the following year. The selection differential is a weighted sum of age-specific differentials that are the covariances between the phenotype and the age-specific relative fitness defined by the individual reproductive value. This enables estimation of weak selection on a multivariate quantitative character in populations with no density regulation by combinations of age-specific linear regressions of individual reproductive values on the traits. Demographic stochasticity produces random variation in fitness components in finite samples of individuals and affects the statistical inference of the temporal average directional selection as well as the magnitude of fluctuating selection. Uncertainties in parameter estimates and test power depend strongly on the demographic stochasticity. Large demographic variance results in large uncertainties in yearly estimates of selection that complicates detection of significant fluctuating selection. The method is illustrated by an analysis of age-specific selection in house sparrows on a fitness-related two-dimensional morphological trait, tarsus length and body mass of fledglings.
Institut des sciences de l'evolution
Evolution of senescence in heterogeneous landscapes
Author(s): Cotto, O, Ronce, O
The current theory of senescence is developed in a very simple ecological and demographic context, with a unique population at equilibrium in a homogeneous habitat. In the wild, species live in a variable environment in space and time, where the assumption of equilibrium is often transgressed. In this study, we use models of quantitative genetics in structured populations in order to investigate the evolution of senescence in a variable environment. Adaptation to local environment depends on phenotypic traits which expression varies with age. We study different scenarios where the environment changes abruptly, gradually or cyclically with time and where the environment is heterogeneous in space with different populations connected by migration. The strength of selection decreases with age, which predicts slower adaptation of traits expressed late in the life cycle, potentially generating stronger senescence in habitats where selection changes in space or in time. This prediction is however complicated by the fact that the genetic variance also increases with age. With numerical calculations, we found that in most cases the rate of senescence is enhanced when the environment varies. Especially, migration between different habitats is a durable source of senescence in heterogeneous landscapes. We also show that the rate of senescence can sometime decrease transiently, when the population is not at equilibrium, with possible implications in experimental evolution and in the study of invasive species. Our results highlight the need to study age-specific adaptation, as a changing environment can impact differently each age-class with different consequences on demography.
School of Biological Sciences
Examining the link between genotype and phentype: mitochondrial gene expression levels across distinct haplotypes, sexes, and ages
Author(s): Camus, F, Wolf, JBW, Morrow, EH, Dowling, DK
Mitochondria are key components of cellular metabolic processing, providing most of the cellular energy required for survival. The small set of genes located within the mitochondria has recently been the subject of much attention by evolutionary biologists, as a groundswell of studies have documented that allelic variance within the mitochondrial DNA (mtDNA) often confers modifications to the phenotype. Mitochondria have been shown to play an active role in the process of ageing, as already proposed by Harman in 1972, and recent studies suggest that some of this allelic variance is even male-specific. Here we use the fruitfly Drosophila melanogaster as a model to advance our understanding of the link between the mitochondrial genotype and phenotype. We sequenced full mitochondrial genomes 13 lines and examine genotype-specific transcriptional profiles of 10 (out of 13) mitochondrial key genes on an isogenic nuclear background. Gene expression patterns are specifically examined for males and females and across age classes. Our goal is to understand the elusive mechanisms that mediate mitochondrially-encoded effects on the phenotype, and to home in on the candidate mutations that cause these effects.
Institute for Zoology
Growth, maintenance and reproduction in ant colonies
Author(s): Schrempf, A, Kramer, B, Klein, A, Scheuerlein, A, Gampe, J, Heinze, J
Organisms have to allocate their available resources into growth, maintenance and reproduction throughout their life, and several life-history studies have documented a trade-off between growth and reproduction. However, the invested energy and costs for maintenance are usually difficult to evaluate in individual organisms. In this study, we monitor single-queen ant colonies of the species Cardiocondyla obscurior throughout their whole life. With this model organism, it is possible to not just measure the allocation of resources into colony growth/maintenance and into sexual reproduction but to gain insights in the time-dependent resolution of the allocation patterns throughout the complete life cycle of a large number of colonies and the consequences on colony fitness and queen lifespan. Specifically, we focus on investment into maintenance, which cannot be measured in whole organisms, and its consequences for subsequent colony survival.
Human evolutionary demography: illustrated with reference to the importance of kin for human reproductive success
Author(s): Sear, R
Evolutionary demographers working on our own species are fortunate: data on humans abounds, both from the real world (including large-scale national datasets collected by demographers and economists and data on small-scale traditional societies collected by demographers), and from the lab (psychological and medical). We also have access to the substantial amount of research done in the social and medical sciences on how to collect, analyse and think about such data. In this talk I will summarise the benefits of the cross-disciplinary approach of human evolutionary demography, which combines data, methods and insights from the social sciences with the theoretical framework of evolutionary biology. I will do this with particular reference to my research on kin influences on demographic outcomes, including child survival and fertility rates. As a social species, interactions with other individuals are important for human fitness. The ‘cooperative breeding’ and ‘pooled energy budget’ models of human social organisation suggest, in fact, that such interactions are essential for human reproductive success. Here I will present results from a comparative project which is investigating the empirical evidence that kin do matter for women’s fitness across a wide range of human populations, including: the analysis of nationally representative datasets from both high and low income countries; a comparative analysis of datasets from traditional, subsistence societies contributed by anthropologists; and psychological experiments. This evidence demonstrates that the presence of kin is often correlated with higher reproductive success, but also that interactions between kin are not always necessarily cooperative. There is also some evidence for local resource competition between kin, and conflicts of interest between affinal kin (those related by marriage).
Indicators of selection pressure, and changes in vital rates at multiple ages: a general method
Author(s): Wensink, M, Wrycza, T, Baudisch, A
Hamilton's (1966) indicators of selection pressure on age-specific additive changes in mortality and fecundity have provided evolutionary biology with the key to study evolutionary effects of age-specific changes in vital rates. However, other indicators of ‘selection pressure’, the sensitivity of fitness to some standardized perturbation of a vital rate, have been proposed and advocated, involving non-additive changes in vital rates (Baudisch 2005), or changes that involve multiple ages (Hamilton 1966, Abrams 1991). In addition, it may be wearisome to obtain an indicator of selection pressure on more complex patterns of change across ages. Realizing that any eventual effect on fitness results not only from selection pressure, but also from the force of perturbation of the vital rates, we develop the calculus to evaluate the effect on fitness of the combination of selection pressure with a perturbation function. We apply this calculus to: 1) show that other indicators of selection pressure can be derived from Hamilton’s ‘elementary’ indicators on additive perturbations; 2) show that choosing an indicator of selection pressure and the appropriate perturbation function is merely a matter of parameterization, rather than a true conceptual difference; 3) propose to use Hamilton’s indicators of additive change for all analyses, since in this way all the biological variation is contained in the perturbation function; and 4) investigate in what direction a resident phenotype may evolve under some given life-history trade-off that produces change in age-patterns of fecundity and/or mortality. Under 4), the central idea is that life-history trade-offs can be presented as perturbation functions that lead to a different perturbation (different magnitude and direction) at different ages. Implications for the evolutionary theory of aging are discussed.
Institute of Integrative Biology
Maternal-age effects on rates of senescence differ between clones of the water flea, Daphnia pulex: causes and consequences
Author(s): Plaistow, SJ, Shirley, C, Harney, ED, Collin, H
Senescence is widely believed to be the evolved consequence of life-history strategies that are themselves shaped by extrinsic mortality. Consequently, evolutionary explanations of senescence are static. But recent studies investigating lifespan extension, genetic intervention and dietary restriction suggest that rates of senescence are plastic and may be regulated by the same mechanisms that underpin life-history plasticity. The Lansing effect refers specifically to plasticity in lifespan that are derived from maternal age effects: offspring from older mothers often senesce at faster rates. Lansing demonstrated this effect in rotifers over 60 years ago, and Lansing effects have now been observed in a broad range of taxa. However, there is still no evolutionary framework for the Lansing effect, or a mechanism explaining how maternal age effects are transmitted and alter offspring life-histories. We repeated Lansing's experiments in D. pulex. Using offspring from the first clutch to set up young maternal lines, and offspring from the fifth clutch to set up old maternal lines, we reared three clones for three generations. Offspring development, life-history and rates of senescence were then compared in the fourth generation. Lansing Effects were observed irrespective of the measure of senescence used, and varied in strength between clones. Offspring from old maternal lines were born larger, grew more, initiated maturation at larger sizes, and had increased early lifetime reproductive effort. However, differences in the growth and maturation decisions of offspring from young and old maternal lines were size-independent, supporting Lansing's assertion that older mothers transmit a non-genetic "ageing factor" to their offspring that alters their life-histories. We propose a novel adaptive explanation for the Lansing effect, and discuss the implications that an interaction between genetic and non-genetic inheritance have life-history evolution and population demography.
Institute of Biology
Ontogenescence and the Barnacle: an experimental examination of early life mortality in the estuarine barnacle Amphibalanus improvisus
Author(s): Dunn, PH, Levitis, DA
Ontogenescence, the decrease in mortality rate experienced during early life, is a nearly universal life-history trait. Among aquatic organisms this high and declining early mortality is generally attributed to extrinsic risks to which developing individuals gradually become more robust through increasing size and speed (Acquisition of Robustness). We examined the stage-specific mortality patterns of larval estuarine barnacle Amphibalanus improvisus in the laboratory, and found that in the absence of the usual environmental risks (predation, washing away, rapid environmental changes), early mortality was still high and generally declining, but was focused around developmental transitions. We further found that even though individuals of these transitional stages are more likely to encounter certain environmental stressors (temperature and salinity shocks), these stages exhibited the lowest tolerance to these stressors. Our results, while not refuting the Acquisition of Robustness hypothesis, lend support to the Transitional Timing Hypothesis, which states that ontogenescence arises because biological transitions are dangerous, and are concentrated early in life. Our data illustrate several ways in which mortality can be concentrated around early-life transitions.